Process for molding composite articles

ABSTRACT

A method of molding composite articles including arranging a pair of matched-tool mold sections in an opposed relationship, each mold section having a rigid housing and a thin, semi-rigid membrane removably and sealably mounted on the housing so as to define a fluid-tight chamber; filling the fluid-tight chambers with a non-compressible backing fluid; and accommodating thermal expansion of the backing fluid by an expansion chamber in fluid communication with the fluid tight chambers. The method further includes defining a mold plenum between the semi-rigid membranes when the mold sections are closed together; inserting reinforcement materials between the semi-rigid membranes prior to closing the mold sections; and injecting and curing a molding fluid after closing the mold sections. A step for regulating the temperature of the backing fluid may be included wherein a system of coils extend within each fluid tight chamber and wherein the coils are connected to an external heater/chiller unit and the coils are operative to circulate a temperature control fluid therethrough.

This application is a divisional application of U.S. patent applicationSer. No. 08/715,533 filed Sep. 18, 1996, now U.S. Pat. No. 5,971,742,which is hereby incorporated herein by reference.

TECHNICAL FIELD

The present invention relates to the manufacture of composite articles,that is, articles typically comprising a fiber reinforcement latticewithin a cured resin matrix. More specifically, the invention relates tomatched-tool molding apparatus suitable for injection molding compositearticles at controlled temperatures with readily replaceable, low-costtool surfaces.

BACKGROUND OF THE INVENTION

Reaction injection molding and resin transfer molding are processeswherein dry fiber reinforcement plys/preforms are loaded in a moldcavity whose surfaces define the ultimate configuration of the articleto be fabricated, whereupon a flowable resin is injected under pressureinto the mold cavity (mold plenum) thereby to saturate/wet the fiberreinforcement plys/preforms. After the resinated preforms are cured inthe mold plenum, the finished article is removed from the mold.

The prior art teaches injection molding apparatus which consist of apair of complementary or “matched” tools which provide these moldingsurfaces, which each tool being carefully machined, for example, from arigid metal which is otherwise relatively nonreactive with respect tothe resin to be used in conjunction therewith. Such matched metal moldsare expensive to fabricate and are necessarily limited to themanufacture of a single article of a given design. Stated another way,even slight changes to the desired configuration of the article to befabricated may necessitate the machining of an entirely new replacementtool.

Additionally, such known metal tools typically have substantial thermalmass which becomes increasingly problematic as the mold temperaturedeviates from the desired process temperatures. In response, such toolsare often provided with an integral system of internal heating and/orcooling tubes or passages through which an externally suppliedheating/cooling fluid may be circulated. However, in accordance withthese prior art designs, the heating/cooling passages are positionedrelative to the tool surfaces so as to leave a minimum spacing ofperhaps 2 inches (5 cm) therebetween to ensure that the resultingarticle will be free of hot and cold lines or bands which mightotherwise be generated in the article as a result of disparateheating/cooling rates during resin cure. This minimum spacing, in turn,inherently limits the ability of these prior art tools to accuratelycontrol temperature during the injection molding process, again,particularly where such processes are exothermic. And temperaturecontrol of the mold plenum becomes further problematic wherevariable-thickness articles are to be fabricated, given that the thickerportions of the article may well polymerize earlier, and will likelyreach higher temperatures, than the thinner portions thereof.

Still further, where matched metal tools are utilized in processesemploying reduced cycle times, the sizable thermal mass of such metaltools can often generate peak temperatures in the range of about 375° F.to about 400° F., resulting in “dry spots” which will likely render thefinished article unusable. Accordingly, such matched metal tools mayhave to be periodically idled for sufficient time to permit the mold tocool to an acceptable operating temperature, thereby substantiallyincreasing the cost of article fabrication using such tools. Finally, atthe other end of the temperature scale, reduced mold temperatures areknown to increase the rate of styrene buildup when used with resinsemploying styrene monomers, thereby precipitating greater frequency ofstyrene build-up removal and associated labor costs and equipmentdown-time, with an associated increase in process cost.

In an attempt to provide increased temperature control whilefacilitating removal of the finished article from the molding apparatus,the prior art teaches a modified molding apparatus wherein one of themold surfaces is defined by a flexible member formed, for example, ofrubber. The other mold surface is still defined by a rigid,thermally-conductive metal tool which may be backed by a pressurizedfluid such as steam whereby curing heat is transferred to the moldcavity for endothermic molding operations. Unfortunately, for suchendothermic processes, heating but one side of the mold cavity may limitflexibility as to surface finish and other characteristics of theresulting article and, further, limit the degree to which resin cure maybe accelerated. Moreover, where such molding apparatus are used inexothermic processes, the resulting heat accelerates deterioration ofthe flexible mold surface, thereby preventing long-term use of the tool.Moreover, such molding apparatus often requires evacuation of the moldplenum prior to injection of the resin therein, thereby rendering useand maintenance of such molding apparatus more complex, and processesemploying such apparatus more time intensive and costly.

What is needed, then, is a matched-tool injection molding apparatusfeaturing replaceable mold surfaces which are easier and less costly tofabricate than known rigid or flexible tools while further offeringincreased temperature control during both endothermic and exothermicprocesses thereby to provide articles of improved quality at lower cycletimes.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an injection moldingapparatus featuring reusable low-cost molding surfaces.

It is another object of the present invention to provide an injectionmolding apparatus featuring enhanced temperature control of its moldingsurfaces, whereby improved control of the mold process and attendantarticle characteristics can be achieved.

Under the present invention, an injection molding apparatus includes apair of mold sections, wherein each mold section itself includes a rigidhousing and a semi-rigid membrane removably mounted to the housing so asto define a fluid-tight chamber therein. The membrane of each moldsection, which, in turn, defines its molding surface, is preferablyformed of an inexpensive composite material such fiberglass orreinforced nylon, or other suitable material; and, in accordance withthe present invention, different membrane materials and/orcharacteristics may be selected for the respective membranes of eachmold section. When the two mold sections are assembled with theirrespective molding surfaces in opposition to one another, a moldingplenum is defined within which to fabricate the desired article. Thus,under the present invention, design changes to the article are readilyaccommodated through alteration or replacement of the low-costmembrane(s). Stated another way, under the present invention, a givenmold section housing may be outfitted with a wide variety of relativelyinexpensive composite membranes useful in the production of compositearticles of different shapes, sizes and characteristics, thereby greatlyreducing tooling costs as compared to the prior art.

In accordance with the present invention, a noncompressible fluid isdisposed within and fills the chamber of each mold section, whereby itsrespective membrane is supported so as to ensure proper dimensioning ofthe finished article while permitting slight dimensional flexing duringresin injection thereby to evenly distribute any injection-pressureloading of the membrane across its entire surface. The latter featuremay prove especially advantageous where a spike in injection pressure isencountered during the resin injection step. As a further advantage,such slight dimensional flexing of the membrane during resin injectionis believed to improve or enhance the flow of resin through the moldplenum. An expansion chamber in fluid communication with the chamber ofone or both mold sections serves to accomodate thermal expansion of themembrane-backing fluid prior to injection of resin into the mold plenum,and subsequent to cure of the finished article, with a valve operatingto isolate the chamber from the expansion chamber during resin injectionand cure.

And, in accordance with another feature of the present invention, thebacking fluid is itself preferably thermally conductive; and the moldingapparatus further includes means in thermal communication with thebacking fluid within one or both of the mold sections for regulating thetemperature of the backing fluid. For example, in a preferredembodiment, the temperature regulating means includes a system of coilsextending within each chamber, and an external heater/chiller unit ofconventional design which is connected to the coil system and isoperative to circulate a temperature control fluid at a predeterminedtemperature therethrough. In this manner, the temperature of the backingfluid and, correlatively, of the molding surface of each mold sectionmay be closely regulated, thereby offering improved characteristics ofthe finished article and/or improved control of process parameters, suchas cure time and temperature. Additional benefits of such temperatureregulation of molding surfaces include, for example, reduced styrenebuild-up, with an attendant reduction in mold down-time and moldmaintenance costs as compared to prior art molding apparatus.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a partially diagrammatic, partially exploded isometric view ofan injection molding apparatus in accordance with the present invention;and

FIG. 2 is a cross-sectional view of the apparatus shown in FIG. 1 alongvertical plane passing through line 2—2 thereof subsequent to assemblyof the upper mold section onto the lower mold section thereof.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to FIG. 1, an exemplary apparatus 10 under the presentinvention for molding a composite article includes a mold assembly 12having an upper mold section 14 and a lower mold section 16 whichdefine, upon assembly of the upper mold section 14 onto the lower moldsection 16 with the aid of locating pins 18 and complementary locatingslots 20, a mold plenum 22 with the matched molding surfaces 24,26thereof. Specifically, the lower and upper mold sections 14,16 eachinclude a rigid housing 28,30 and a relatively thin, semi-rigid membrane32,34 which is removably and sealably secured to the espective housing28,30 along the membrane's peripheral edge as by a clamping ring 36.Thus assembled, the housings 28,30 and membranes 32,34 of each moldsection 14,16 cooperate to define fluid-tight chambers 38,40 therein.

In accordance with one feature of the present invention, each membrane32,34 is itself preferably formed of a composite overlay which, in itsmost elegant form, may simply comprise splash off of a blank of thearticle to be fabricated. And, while each membrane 32,34 mayconveniently be formed of fiberglass or reinforced nylon, the presentinvention contemplates use of semi-rigid membranes 32,34 fabricated fromother suitable materials such as light sheet metal, which membranes32,34 may be conveniently and cheaply fabricated, shaped and reshaped ina pressure chamber in a manner known to those skilled in the art. Inthis regard, it is noted that the present invention contemplates use ofeither the same or different materials for the respective membranes32,34 of each mold section 14,16 depending, for example, upon thedesired characteristics of the sheet (e.g., its thermal conductivity,formability, and usable life), the desired characteristics of thefabricated article (e.g., surface finish and gloss), and/or overallprocess parameters (e.g., resin injection pressures, resin cure time andmold assembly cycle time).

The fluid-tight chambers 38,40 defined within each mold section 14,16are completely filled with a substantially non-compressibleheat-conductive fluid 42 supplied by a fluid supply network 44 prior toinjection of resin into the mold plenum 22. The fluid 42 within eachchamber 38,40 thereby provides support for each membrane 32,34 incompression during resin injection in a manner to be further describedbelow.

In the preferred embodiment shown in FIG. 1, the membrane-backing fluid42 is conveniently tap water which is supplied by the network 44 to theupper and lower mold assemblies 14,16 as through respective inletcontrol valves 46 and quick connect couplings 48. Other suitable backingfluids useful over different operating ranges (e.g., having highervaporization temperatures) will be known to those skilled in the art. Apressure gauge 50 may be employed downstream of each inlet valve 46 tomonitor the flow rate of backing fluid 42 into the chamber 38,40 of eachmold section 14,16. To facilitate the filling and emptying of eachchamber 38,40, each mold section 14,16 has a vent 52 through which airwithin each chamber 38,40 may escape upon the filling thereof withbacking fluid 42. Once filled, each chamber's vent 52 is sealed with avent plug 54, thereby imparting requisite rigidity to each moldsection's membrane/molding surface 24,26.

As seen in FIG. 2, wherein the relative dimensions of, for example, themembranes 32,34 and mold plenum 22 are exaggerated for ease ofillustration, each mold section 14,16 includes a system ofheating/cooling coils 56 extending within the fluid-tight chamber 38,40thereof which are themselves coupled via quick connect couplings 58 toan external heater/chiller unit 60 of conventional design. As such, thecoils 56 operate in conjunction with the heater/chiller unit 60 toprecisely regulate the temperature of the backing fluid 42 and, hence,the molding surface 24,26 of each membrane 32,34 throughout theinjection molding process. And, while the coils are illustrated in FIG.2 as being located proximate to the back side of the composite membrane,under the present invention, the thermal conductivity of the backingfluid 42 enables substantial design variation with respect to placementof the coils 56 within the chamber 38,40 of each mold section 14,16which, in turn, facilitates use of a given mold section housing 28,30and coil system 56 with a wide variety of membranes 32,34. Indeed, underthe present invention, while the membranes 32,34 of the exemplaryapparatus 10 are shown in FIG. 2 as being of relatively uniformthickness, the efficiency with which mold temperature may be controlledunder the present invention permits the use of variable-thicknessmembranes 32,34, as may be desirable, for example, when providing thefinished article with reinforcement ribs.

To the extent that the backing fluid 42 with which each mold section14,16 is filled is supplied at a temperature different from the desiredprocess temperature, the fluid supply network 44 further includes alow-pressure expansion chamber 62. Thus, upon subsequent heating orcooling of each mold section 14,16 to the desired temperature, anyresulting thermal expansion of the backing fluid 42 within each chamber38,40 will be accommodated by the expansion chamber 62, therebypreventing distortion and/or deleterious stress on the membranes 32,34.

Returning to the Drawings, an injection sprue 64 may be seen in FIG. 2as extending through the upper mold section 14 to provide a pathwaythrough which a desired thermoset resin from a resin supply 66 may beinjected under pressure by a suitable pump 68 into the mold plenum 22.The number and placement of such sprues 64 depends upon theconfiguration and desired characteristics of the article to be molded,and the flow characteristics of the resin employed, in a manner known tothose skilled in the art. In this regard, it will be seen that a seriesof small vents 70 is provided between the opposed clamping rings 36 ofthe upper and lower mold sections 14,16 through which trapped air maybleed to atmosphere during injection of the resin into the mold plenum22.

In accordance with another feature of the present invention, theexemplary molding apparatus 10 further includes a mechanism indicatedgenerally by reference numeral 72 on the lower mold section 16 forvibrating the mold assembly 12 or, at a minimum, the backing fluid 42contained in the lower mold section 16. Vibration of the mold assembly12/backing fluid 42 during injection of the resin is believed tofacilitate resin flow through the mold plenum 22, as well as to improvesaturation and wetting of fiber reinforcement preforms (not shown)situated therein.

In accordance with the present invention, the exemplary moldingapparatus shown in the Drawings may be used as follows: one or morefiber reinforcement preforms are laid within the mold cavity defined bythe “female” molding surface 26 of the lower mold section 16. The uppermold section 14 is thereafter lowered onto the lower mold section 16 soas to engage each locating pin 18 with its respective locating slot 20(where desired, the upper mold section 14 may then be secured to thelower mold section 16 as through the use of suitable clamps, not shown).Each mold section 14,16 is then connected to the backing fluid (water)supply network 44, and its respective vent 52 is opened and inlet valve46 is operated, thereby to completely fill the chamber 38,40 thereinwith water.

Once the chambers 38,40 are completely filled, each mold section vent 52is sealed with its respective vent plug 54 and the heater/chiller unit60 operated to bring each mold section 14,16 to the desired processtemperature. The inlet valve 46 to each mold section 14,16 is thereafterclosed to isolate its respective chamber 38,40 from the fluid supplynetwork's expansion chamber 62 (which otherwise has accommodated anythermal expansion of the backing fluid 42 during temperaturenormalization). By way of example only, where the resin to be injectedis a thermoset polyester or vinylester resin, the desired operatingtemperature necessary to provide desired flow characteristics for agiven thermoset polyester or vinylester resin has been shown to be 140°F. to about 150° F.

The desired resin is thereafter injected under pressure into the moldplenum 22 through the injection sprew 64. Where the membranes areformed, for example, of fiberglass with a nominal thickness of perhapsabout 0.375 inches (0.95 cm), a typical injection pressure used ininjecting a thermoset polyester or vinylester resin having a viscositybetween of between about 400 and 500 centipoise into the mold plenum 22is preferably less than about 100 psig (690 kPa) and, most preferably,less than about 60 psig (410 kPa). Of course, the optimal flow rate atwhich the resin is injected is based upon a number of factors well knownto those skilled in the art.

Once the mold plenum 22 is completely filled with resin, as visuallyconfirmed by discharge of resin through the air bleeds formed in theclamping rings 36 of each mold section 14,16, the injection of resinceases. The temperature of each molding surface 24,26 is thereafterregulated via operation of the heater/chiller unit 60 to thereby providean optimum cure rate with which to obtain the desired surface finishand/or other desired characteristics of the finished article, or tootherwise optimize the molding process. The mold sections 14,16 arethereafter separated, and the finished article removed from the moldcavity in a conventional manner.

In accordance with another feature of the present invention, due to thesemi-rigid character of the lower mold section's membrane 34, themembrane 34 will dimensionally flex slightly during resin injection asthe backing fluid 42 distributes the resulting injection pressure loadacross the entire surface of the membrane 34. In this manner, thesemi-rigid membrane 34 avoids deleterious stress concentration on itsmolding surface 26 during resin injection. Indeed, the slight flexing ofthe molding surfaces 24,26 of one or both membranes 32,34 during resininjection is believed to further improve or enhance the flow of resinthrough the mold plenum 22, which effect may be further enhanced bydeliberately pulsing the injected resin, all without deleterious impacton the molding tools (the membranes 32,34).

While the preferred embodiments of the invention have been disclosed, itshould be appreciated that the invention is susceptible of modificationwithout departing from the spirit of the invention or the scope of thesubjoined claims. For example, while the preferred embodiment employsmembrane-backing fluid 42 which is itself fully contained within thechamber 38,40 of each mold section 14,16, to be heated or cooled byheater/chiller unit 60 via coils 56, the present invention contemplatesthe use of a closed loop temperature regulating system wherein thebacking fluid 42 is itself circulated between each mold section'sinternal chamber 38,40 and the heater/chiller unit 60.

I claim:
 1. A method of molding a composite article comprising thesteps: a. arranging in a spaced apart opposed relationship a first moldsection comprising a first semi-rigid membrane removably mounted to afirst rigid housing to define a first fluid tight chamber therein and asecond mold section comprising a second semi-rigid membrane removablymounted to a second rigid housing to define a second fluid tight chambertherein; b. defining a mold plenum between the first and secondsemi-rigid membranes when the first and second mold sections are closedtogether to mold articles; c. filling the first and second fluid tightchambers with a substantially non-compressible backing fluid to supporteach membrane during injection of a molding fluid; d. accommodatingthermal expansion of the backing fluid by an expansion chamber in fluidcommunication with the first and second fluid tight chambers; e.inserting reinforcement materials between the first and secondsemi-rigid membranes prior to closing the first and second mold sectionstogether to mold articles; f. closing the first and second mold sectionstowards each other such that the reinforcement materials are sandwichedin the mold plenum; g. rigidly retaining the first and second moldsections together with the reinforcement materials sandwiched in themold plenum; h. injecting molding fluid under pressure into the moldplenum; i. distributing a resulting injection pressure by the backingfluid; and j. curing the molding fluid by heating at least one of thebacking fluid to produce a molded article.
 2. The method of claim 1further comprising isolating said expansion chamber during at least oneof said injecting molding fluid and said curing said molded article. 3.The method of claim 1 further comprising regulating the temperature ofthe backing fluid by a system of coils extending within each fluid tightchamber wherein the coils are connected to an external heater/chillerunit and the coils are operative to circulate a temperature controlfluid therethrough.
 4. The method of claim 3 further comprising the stepof regulating the temperature of the molding surface of the semi-rigidmembrane of each mold section.
 5. The method of claim 1 comprising thestep of pulsing the molding fluid during injecting of molding fluid tothe mold plenum.
 6. The method of claim 1 comprising the step ofvibrating a mold assembly formed by closing the mold sections togetherduring injecting of molding fluid to the mold plenum.